Add contribution guidelines
Add contribution guidelines inspired by
- https://tails.boum.org/contribute/code/
- https://tails.boum.org/contribute/git/
- https://tails.boum.org/contribute/merge_policy/
Most important thing is to document how to work with branches related to tickets in the issue tracker so merging gets easier.
(from redmine: created on 2012-05-24)
Edited by Jerome Charaoui